The present invention relates to a system and a method for controlling a lean-burn internal combustion engine, and more particularly, to minimizing NOx emissions during a purge of the LNT.
Internal combustion engines are typically coupled to an emission control device known as a three-way catalytic converter (TWC) designed to reduce combustion by-products such as carbon monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOx). Engines can operate at air-fuel mixture ratios lean of stoichiometry, thus improving fuel economy. For lean engine operation, an additional three-way catalyst commonly referred to as a Lean NOx Trap (LNT), is usually coupled downstream of an upstream three-way catalyst. The LNT, like the TWC, stores exhaust gas constituents such as, for example, nitrogen oxides, NOx, when the engine is operating at a lean air-fuel ratio, and reduces (purges) them when the engine is operating at a rich or stoichiometric air-fuel ratio.
Because continued lean operation will ultimately saturate the LNT with NOx, the prior art teaches periodically varying the air-fuel ratio from a nominally lean setting to a rich setting, during which stored NOx are released from the LNT and reduced by the available hydrocarbons and carbon monoxides in the enriched operating condition.
The inventors herein have recognized that during the purge, some of the released NOx is not reduced, and is therefore emitted into the atmosphere. The inventors have further recognized that the LNT temperature raises during the purge due to the exothermic reaction created by the reduction of NOx by CO in the rich air fuel mixture entering the LNT, and once the temperature exceeds a predetermined threshold, the LNT""s NOx storage capacity decreases with increasing temperature. Therefore, the inventors recognized that this decrease in the NOx storage capacity as the LNT temperature is increasing, would cause the NOx to be displaced from the LNT until the absorbed NOx is equivalent to the reduced LNT capacity at higher temperature. The NOx thus displaced would exit the LNT as emissions, and therefore the performance of the LNT would be degraded.
In solving the above problem, a system and a method are provided for decreasing NOx emissions during an LNT purge by active temperature control of the LNT.
In carrying out the above solution, features and advantages of the present invention, a system and a method for improving a performance of an exhaust gas aftertreatment device during a purge, the device coupled downstream of a lean-burn internal combustion engine, include: providing an indication of a device operating condition, wherein said condition is reached when a capacity of the device to store an exhaust gas component decreases with increases in a temperature of the device; and in response to said indication, adjusting an operating parameter, thereby decreasing said device temperature.
An advantage of the present invention is improved emission control, due to the fact that maintaining the temperature of the LNT in the region where NOx storage capacity does not reduce with increases in temperature, reduces NOx emissions during purge.